Synergies and prospects for early resolution of the neutrino mass ordering.

The measurement of neutrino mass ordering (MO) is a fundamental element for the understanding of leptonic flavour sector of the Standard Model of Particle Physics. Its determination relies on the precise measurement of [Formula: see text] and [Formula: see text] using either neutrino vacuum oscillations, such as the ones studied by medium baseline reactor experiments, or matter effect modified oscillations such as those manifesting in long-baseline neutrino beams (LB[Formula: see text]B) or atmospheric neutrino experiments. Despite existing MO indication today, a fully resolved MO measurement ([Formula: see text]) is most likely to await for the next generation of neutrino experiments: JUNO, whose stand-alone sensitivity is [Formula: see text], or LB[Formula: see text]B experiments (DUNE and Hyper-Kamiokande). Upcoming atmospheric neutrino experiments are also expected to provide precious information. In this work, we study the possible context for the earliest full MO resolution. A firm resolution is possible even before 2028, exploiting mainly vacuum oscillation, upon the combination of JUNO and the current generation of LB[Formula: see text]B experiments (NOvA and T2K). This opportunity is possible thanks to a powerful synergy boosting the overall sensitivity where the sub-percent precision of [Formula: see text] by LB[Formula: see text]B experiments is found to be the leading order term for the MO earliest discovery. We also found that the comparison between matter and vacuum driven oscillation results enables unique discovery potential for physics beyond the Standard Model.